Conventionally, a collision damage reduction apparatus has been known that makes the brake of a vehicle operate automatically when time to collision (TTC) becomes a predetermined time or shorter where the TTC is a time until the vehicle collides with an obstacle, which is calculated based on the distance to the detected obstacle and the relative speed of the vehicle with respect to the obstacle (see, for example, Patent Document 1)
An obstacle detection unit, such as a radar (a millimeter-wave radar, a laser radar, etc.) or a camera, included in such a collision damage reduction apparatus has a limit in its detection range of obstacles. Specifically, if the distance to an obstacle is very short (for example, the distance to an obstacle is 2 m or shorter), the obstacle comes into the dead angle of the obstacle detection unit, and the obstacle is not detected by the obstacle detection unit. Therefore, by continuing a brake-operational state until the vehicle stops even if the obstacle has gone out of detection, a collision with the obstacle gone out of detection can be avoided appropriately.
On the other hand, it is desirable to release a brake-operational state before the vehicle stops, for cases where, for example, an erroneously detected obstacle, such as a manhole cover on a road surface, has gone out of detection; after an obstacle (a preceding vehicle) has gone out of detection, the obstacle makes a lane change or the like to avoid a collision; and the like.
Thereupon, a technology has been proposed that calculates an estimated distance to the obstacle if an obstacle has gone out of detection, and if the estimated distance is less than or equal to 0, releases the brake-operational state (see, for example, Patent Document 2). Namely, if the estimated distance is less than or equal to 0, it can be considered that the vehicle reaches the obstacle that has gone out of detection. Therefore, if a collision is not detected in this case, it can be determined that a collision with the obstacle gone out of detection is avoided, or the brake operation was made due to an erroneous detection of the obstacle. Therefore, if the estimated distance is less than or equal to 0, by releasing the brake-operational state, it is possible to have the vehicle resume normal traveling, base on an appropriate determination that a collision with the obstacle gone out of detection is avoided, or the brake operation was made due to an erroneous detection of the obstacle, and the brake-operational state to avoid a collision with the obstacle gone out of detection has been continued until the determination is made.